The difference is that when SINGING, a vocalist
pronounces things differently than when spoken. For
example, we don't (or shouldn't at any rate) sing the R
at the end of a syllable. So, AMERICA is "A-meh-i-ca"
so it doesn't sound like "A- merrrrr-i-ca"

When singing the word EXCELSIS, common practice is to
sing EGG-SHELL-CEASE. It make the word sound correct to
the listener.

Another Latin example is the initial V sound. Correct
Latin pronounces it like W, but that doesn't work with
music. The pronunciations used in liturgical music have
taken hold in liturgical Latin, supplanting classical
use.